The present invention relates generally to paper making and, more particularly, to the construction of a press section of a paper machine.
The present invention is particularly directed to improvements in press sections which include, in the direction of web passage through the press section, at least three successive, separate dewatering press nips, i.e. press nips formed by their own respective pairs of press rolls, and wherein the web is a closed draw as it travels between the nips. The web is dewatered through both of its faces at least in the first one of the press nips formed between two preferably hollow-faced press rolls. The press section further includes at least one upper fabric and lower fabrics. Of these fabrics, the first fabrics in the direction of web run are water-receiving press fabrics arranged so that one of them acts as a press fabric in the first press nip while the other acts as a press fabric both in the first as well as in the second press nip. Of the second fabrics in the direction of web run, the upper fabric is a water-receiving press fabric while the second lower fabric is a substantially non-water receiving transfer fabric or belt on which the web is transferred after the nip as a closed draw to the drying section which follows the press section.
Dewatering of a paper web by evaporation consumes large amounts of energy and is therefore a costly and uneconomical operation. For this reason, it is usually attempted to remove as much water as possible from the web by mechanical means before the web reaches the drying section of the paper machine. The last stage where water is mechanically removed from the web in a paper machine is the press section where water is removed by pressing the web between rolls. It is well known that water will be more readily removed from the web when the temperature of the water is elevated since the viscosity of the water and the springback coefficient or coefficient of compression elasticity of the web are thereby reduced as is the surface tension.
As the rates of production of paper machines increase, one of the major bottle-necks which limit further increased production rates is the existence of open draws of the web after the press section, either from the press section to the drying section or in the first free spaces in the drying section itself.
The most common conventional arrangement for dewatering fibrous webs, in particular paper and cardboard webs, is one where the web is passed through a press nip formed between two rolls situated in facing relationship to each other. As is well known, one or two press fabrics are used in such dewatering nips, the fabrics carrying the water drained from the web away from the nip and also functioning to carry the web forwardly after the press nip.
A fully closed, compact press section, designated "Sym-Press", available from Valmet Oy of Finland, the assignee of the instant application, is disclosed in Finnish Annoucement Publication No. 50,651. This closed, compact press section utilizes a press-suction roll and a rock or stone roll which may result in certain drawbacks. For example, the perforations formed in the suction rolls reduce the strength of the mantle of the suction rolls thereby requiring the use of special metal alloys in the construction of such rolls as well as requiring that the thickness of the mantles of the suction rolls be relatively great.
Moreover, although it is highly desirable that the press-suction rolls be provided with a variable-crown arrangement, it is not always possible to do so due to the perforations in the mantle of the suction roll and/or since the interior of the press-suction roll is occupied by suction equipment to the extent that the space available for variable-crown equipment is not sufficient.
Prior to the advent of closed and compact press sections, such as the Sym-Press and Sym-Press II press sections of Valmet Oy, open press sections were commonly used wherein each press nip was formed between its own respective pair of press rolls. In this connection, reference is made by way of example to U.S. Pat. Nos. 2,694,348 (E.D. Beachler), 3,268,390 (D.E. Ely), 4,219,383 (P.J. Valkama). One of the reasons why these open press sections are being replaced by fully closed and compact press sections is that a compact press section, e.g. the Sym-Press II press section, requires considerably less space than open press sections. Moreover, the open press sections of the type described in the above mentioned U.S. patents have certain additional drawbacks. For example, the web tends to be rewetted after passing through one or more of the press nips, such rewetting being particularly detrimental between the second and third press nips, especially in the case where thin paper grades are being manufactured.
Attempts have been made to increase the dewatering capacity of a nip press by increasing the nip pressure. However, a certain line pressure is eventually reached beyond which any increase in the nip pressure is no longer desirable since the structure of the web and the press fabrics cannot withstand such increased compression pressures.
The dewatering capacity of a nip press can be increased by lengthening the zone of the roll nips through the use of larger diameter rolls and softer press fabrics. However, a limit is again reached using even this technique.
Accordingly, for these and other reasons, so-called extended-nip presses have recently been proposed. In this connection, reference is made, by way of example, to U.S. Pat. Nos. 3,808,092, 3,808,096, 3,840,429, 3,970,515, 4,210,624, and 4,229,253, to G.B. Pat. Appl. No. 20 57 027, as well to Finnish Patent Application Nos. 82-3500, 83-0995, and 83-1028, the Finnish applications being assigned to the assignee of the instant application.
In Finnish Patent Application No. 82-3187, corresponding to U.S. Application Ser. No. 531,297 filed Sept. 12, 1983, now U.S. Pat. No. 4,526,655, assigned to the assignee of the instant application, a press section is disclosed which constitutes one of the starting points of the present invention. The press section disclosed in said application is an open press section and includes two upper fabrics and two lower fabrics, of which the first fabrics in the direction of web run are water-receiving press fabrics arranged so that one of them acts as a press fabric in the first press nip while the other acts as a press fabric both in the first as well as in the second press nip. Of the second fabrics in the direction of web run, the upper fabric or the lower fabric is a water-receiving press fabric while the other is a substantially non-water receiving transfer fabric on which the web is transferred after the third nip to the drying section following the press section as a closed draw.
Objectives of the press section disclosed in application Ser. No. 531,297, now U.S. Pat. No. 4,526,655, are as follows:
to provide a fully closed draw of the web from the forming wire to the drying fabric,
to provide a press section in which press suction rolls are not required, although it is noted that in some cases it is possible or even desirable to use one or more suction rolls for certain purposes, preferably not as press-suction rolls,
to provide a press section in which sufficient space is provided between the press nips to allow for the positioning in connection with the press rolls of such equipment as loading devices and doctors, as well as guide and reconditioning devices for the various fabrics, and to allow ease of operation in maintenance of such equipment as well as removal of broke,
to provide a press section by means of which a higher dry matter content is obtained or, alternatively, in which it is possible to use pulps of lesser quality relative to that required in conventional paper machines,
to provide a press section which avoids or substantially reduces the problems of vibration which exist in conventional press sections and, in particular, in conventional compact press sections, and
to provide a press section wherein the same construction can have a lower weight than is the case in the prior art without incurring any risk of additional vibrations.